In power conversion applications, constant on time (COT) control is widely used in DC-DC converters since COT control has a simple control loop with fast dynamic response. Traditionally, the COT control for a DC-DC converter is a variable frequency control. Specially, a switch of the DC-DC converter having COT control has a constant on-time, and the duty cycle of the switch is regulated through changing the switching frequency of the switch. Thus, an output voltage of the DC-DC converter is regulated through changing the duty cycle of the switch. For a better stability of the system, in an improved COT control, the DC-DC converter is set to have a constant operation frequency and the duty cycle of the switch is regulated with the changes varied in the input voltage and the output voltage of the DC-DC converter.
As shown in FIG. 1, a DC-DC converter 50 having a COT control circuit is illustrated. The COT control circuit comprises a comparing circuit 51, an on time generator 52 and a logic circuit 53. The on time generator 52 is used to generate an on time signal TON based on an output voltage signal VOUT and an input voltage signal VIN of the DC-DC converter 50. And the operating frequency of the DC-DC converter 50 is set by a resistor and a capacitor of the on time generator 52. Generally, the on time generator 52 is integrated on a monolithic chip. Thus, it is hard to flexibly change the operating frequency of the DC-DC converter 50 in different operation conditions due to the integrated resistor and capacitor of the on time generator 52.
Therefore, it is desired to have a solution for flexibly changing the operating frequency of the DC-DC converter with COT control.